1. Field of the Invention
This invention relates to methods of, and apparatus for, surgery of the cornea, and more particularly to a laser-based method and apparatus for corneal surgery.
2. Related Art
Art Related to the Inventive Method and Apparatus for Surgery
The concept of correcting refractive errors by changing the curvature of the eye was brought forth early on, as illustrated in the notable mechanical methods pioneered by J. Barraquer. These mechanical procedures involve removal of a thin layer of tissue from the cornea by a micro-keratome, freezing the tissue at the temperature of liquid nitrogen, and re-shaping the tissue in a specially designed lathe. The thin layer of tissue is then re-attached to the eye by suture. The drawback of these methods is the lack of reproducibility and hence a poor predictability of surgical results.
With the advent of lasers, various methods for the correction of refractive errors have been attempted, making use of the coherent radiation properties of lasers, and the precision of the laser-tissue interaction. A CO2 laser was one of the first to be applied in this field. Peyman, et al., in Ophthalmic Surgery, vol. 11, pp. 325-9, 1980, reported laser burns of various intensity, location, and pattern were produced on rabbit corneas. Recently, Horn, et al., in the Journal of Cataract Refractive Surgery, vol. 16, pp. 611-6, 1990, reported that a curvature change in rabbit corneas had been achieved with a Co:MgF2 laser by applying specific treatment patterns and laser parameters. The ability to produce burns on the cornea by either a CO2 laser or a Co:MgF2 laser relies on the absorption in the tissue of the thermal energy emitted by the laser. Histologic studies of the tissue adjacent to burn sites caused by a CO2 laser reveal extensive damage characterized by a denaturalized zone of 5-10 microns deep and disorganized tissue region extending over 50 microns deep. Such lasers are thus ill-suited to corneal laser surgery.
In U.S. Pat. No. 4,784,135, Blum et al. discloses the use of far-ultraviolet radiation of wavelengths less than 200 nm to selectively remove biological materials. The removal process is claimed to be by photoetching without requiring heat as the etching mechanism. Medical and dental applications for the removal of damaged or unhealthy tissue from bone, removal of skin lesions, and the treatment of decayed teeth are cited. No specific use for cornea surgery is suggested, and the indicated etch depth of 150 microns is too great for most corneal surgery purposes. Further, even though it is suggested in this reference that the minimum energy threshold for ablation of tissue is 10 mJ/cm2, clinical studies have indicated that the minimum ablation threshold for excimer lasers at 193 nm for cornea tissue is about 50 mJ/cm2.
In U.S. Pat. No. 4,718,418, L""Esperance, Jr. discloses the use of a scanning laser characterized by ultraviolet radiation to achieve controlled ablative photode-composition of one or more selected regions of a cornea. According to the disclosure, the laser beam from an excimer laser is reduced in its cross-sectional area, through a combination of optical elements, to a 0.5 mm by 0.5 mm rounded-square beam spot that is scanned over a target by deflectable mirrors. (L""Esperance has further disclosed in European Patent Application No. 151869 that the means of controlling the beam location are through a device with a magnetic field to diffract the light beam. It is not clear however, how the wave front of the surgical beam can be affected by an applied magnetic to any practical extent as to achieve beam scanning.) To ablate a corneal tissue surface with such an arrangement, each laser pulse would etch out a square patch of tissue. Each such square patch must be placed precisely right next to the next patch; otherwise, any slight displacement of any of the etched squares would result in grooves or pits in the tissue at the locations where the squares overlap and cause excessive erosion, and ridges or bumps of unetched tissue at the locations in the tissue where the squares where not contiguous. The resulting minimum surface roughness therefore will be about 2 times the etch depth per pulse. A larger etch depth of 14 microns per pulse is taught for the illustrated embodiment. This larger etch depth would be expected to result in an increase of the surface roughness.
Because of these limitations of laser corneal surgery systems, it is not surprising that current commercial manufactures of excimer laser surgical systems have adopted a different approach to corneal surgery. In U.S. Pat. No. 4,732,148, L""Esperance, Jr. discloses a method of ablating cornea tissue with an excimer laser beam by changing the size of the area on the cornea exposed by the beam using a series of masks inserted in the beam path. The emitted laser beam cross-sectional area remains unchanged and the beam is stationary. The irradiated flux and the exposure time determines the amount of tissue removed.
A problem with this approach is that surface roughness will result from any local imperfection in the intensity distribution across the entire laser beam cross-section.
Furthermore, the intended curvature correction of the cornea will deviate with the fluctuation of the laser beam energy from pulse to pulse throughout the entire surgical procedure. This approach is also limited to inducing symmetric changes in the curvature of the cornea, due to the radially symmetrical nature of the masks. For asymmetric refractive errors, such as those commonly resulting from cornea transplants, one set of specially designed masks would have to be made for each circumstance.
Variations of the above technique of cornea ablation have also been developed for excimer lasers. In U.S. Pat. No. 4,941,093, Marshall et al. discloses the use of a motorized iris in a laser beam path to control the beam exposure area on the cornea. In U.S. Pat. No. 4,856,513, Muller discloses that re-profiling of a cornea surface can be achieved with an erodible mask, which provides a pre-defined profile of resistance to erosion by laser radiation. This method assumes a fixed etch rate for the tissue to be ablated and for the material of the erodible mask. However, etch characteristics vary significantly, depending on the type of the materials and the local laser energy density. The requirements of uniformity of laser intensity across the beam profile and pulse to pulse intensity stability, as well as limitation of the technique to correct symmetric errors, also apply to the erodible mask method.
Another technique for tissue ablation of the cornea is disclosed in U.S. Pat. No. 4,907,586 to Bille et al. By focusing a laser beam into a small volume of about 25-30 microns in diameter, the peak beam intensity at the laser focal point could reach about 1012 watts per cm2. At such a peak power level, tissue molecules are xe2x80x9cpulledxe2x80x9d apart under the strong electric field of the laser light, which causes dielectric breakdown of the material. The conditions of dielectric breakdown and its applications in ophthalmic surgery had been described in the book xe2x80x9cYAG Laser Ophthalmic Microsurgeryxe2x80x9d by Trokel. Transmissive wavelengths near 1.06 microns and the frequency-doubled laser wavelength near 530 nm are typically used for the described method. The typical laser medium for such system can be either YAG (yttrium aluminum garnet) or YLF (yttrium lithium fluoride). Bille et al. further discloses that the preferred method of removing tissue is to move the focused point of the surgical beam across the tissue. While this approach could be useful in making tracks of vaporized tissue, the method is not optimal for cornea surface ablation. Near the threshold of the dielectric breakdown, the laser beam energy absorption characteristics of the tissue changes from highly transparent to strongly absorbent. The reaction is very violent, and the effects are widely variable. The amount of tissue removed is a highly non-linear function of the incident beam power. Hence, the tissue removal rate is difficult to control. Additionally, accidental exposure of the endothelium by the laser beam is a constant concern. Most importantly, with the variation in the ablated cross-sectional area and the etch depth, sweeping the laser beam across the cornea surface will most likely result in groove and ridge formation rather than an optically smooth ablated area.
Other problems that occur with some of the prior art systems result from the use of toxic gases as the lasing material. This is particularly a problem with excimer lasers, which are frequently used in health clinic and hospital environments.
An important issue that is largely overlooked in all the above-cited references is the fact that the cornea is a living organism. Like most other organisms, corneal tissue reacts to trauma, whether it is inflicted by a knife or a laser beam. Clinical results have showed that a certain degree of haziness develops in most corneas after laser refractive surgery with the systems taught in the prior art. The principal cause of such haziness is believed to be surface roughness resulting from grooves and ridges formed while laser etching. Additionally, clinical studies have indicated that the extent of the haze also depends in part on the depth of the tissue damage, which is characterized by an outer denatured layer beneath which is a more extended region of disorganized tissue fibers. Another drawback due to a rough corneal surface is related to the healing process after the surgery: clinical studies have confirmed that the degree of haze developed in the cornea correlates with the roughness at the stromal surface.
For reliable ablation results, a current commercial excimer laser corneal surgery system operates at about 150-200 mJ/cm2. The etch depth at 193 nm is about 0.5 microns per pulse, and the damage layer is about 0.3 microns deep. Light scattering from such a surface is expected.
It is therefore desirable to have a method and apparatus for performing corneal surgery that overcomes the limitations of the prior art. In particular, it is desirable to provide an improved method of cornea surgery which has accurate control of tissue removal, flexibility of ablating tissue at any desired location with predetermined ablation depth, an optically smooth, finished surface after the surgery, and a gentler surgical beam for laser ablation action.
The present invention provides such a method and apparatus. The invention resolves the shortcomings of the current corneal surgical systems, including the use of toxic gases, limitations stemming from correcting only symmetric errors in the case of excimer laser systems, the extensive damage caused by Co:MgF2 and CO2 laser systems, and the uncertainty of the etch depth in the case of YAG or YLF laser systems.
Art Related to the Scanner-Amplifier Laser Invention
The control of laser beam positioning has become a key element in many fields of applications, such as image processing, graphic display, materials processing, and surgical applications involving precision tissue removal.
A general overview of the topic is given in xe2x80x9cA Survey of Laser Beam Deflection Techniquesxe2x80x9d, by Fowler and Schlafer, Proceedings of IEEE, vol. 54, no. 10, pages 1437-1444, 1966.
U.S. Pat. No. 3,432,771 to Hardy et al. issued Mar. 11, 1969, disclosed an apparatus for changing the direction of a light beam in an optical cavity. The cavity consists of a focussing objective located between two reflectors, such as curved mirrors. The relative position of one center of curvature with respect to the other center of curvature can be controlled by positioning of one of the mirrors. Points on the reflectors are located at the object and the image positions for the objective. When the active medium is suitably excited, the orientation of the lasing mode, and hence the position of the spots of light, is determined by the effective angular positions of the reflectors.
U.S. Pat. No. 3,480,875 to Pole, issued Nov. 25, 1969, disclosed a laser cavity which was set up between a pair of plane mirrors. At least one active laser element is located between the mirrors. A pair of lens systems are positioned between the mirrors so that they have a common focal plane between them. A Kerr cell, polarizers, and a compensator suppress light oscillation along certain reflector paths within the cavity, thereby setting up preferred modes of oscillation along other paths. Laser emission occurs along the preferred paths.
U.S. Pat. No. 3,597,695 to James E. Swain, issued Aug. 3, 1971, disclosed an apparatus for amplifying laser light by multiple passes through a lasing material in a single laser cavity. A single amplifier stage achieved what had been accomplished by several stages. This is accomplished by a switching mechanism which directs a laser beam into and out of the cavity at selected time intervals, thereby enabling amplification of low intensity laser pulses to an energy level near the damage limits of the optical components of the system.
U.S. Pat. No. 4,191,928 to John L. Emmett, issued Mar. 4, 1980, disclosed a high energy laser system using a regenerative amplifier which relaxes all constraints on laser components other than the intrinsic damage level of matter, so as to enable use of available laser system components. This can be accomplished by use of a segmented component spatial filter.
Many techniques have been developed for controlling the laser beam direction. For the purpose of this invention, this discussion will be limited to the speed, accuracy, and the scan angle range of different devices used in a random access mode.
Galvanometer mirror scanners have a large scan angle range. However, the mechanical response due to the balance of the coil and the applied magnetic field is limited to a few hundred hertz. The settling time and oscillation about the equilibrium point further limits the accuracy attainable with such devices.
Mirrors positionable with piezo actuators are capable of an accurate hunt-free movement response of up to tens of kilohertz, depending on the design of the mounts. The typical scan angle is on the order of a few milli-radians. Methods to enhance the scan angle have been proposed by J. Schlafer and V. J. Fowler, xe2x80x9cA Precision, High Speed, Optical Beam Scannerxe2x80x9d, Proceedings, International Electron Devices Meeting, 1965. In their report, multiple scanning piezo-mirrors where used to intercept a laser beam, such that the scan angle of each scanner contributes to the total effect, which is the sum of all scan angles. This device requires many individual scanner units, which multiplies in economic cost with the number of units. The mirror size also limits the number of units that can be used before the beam will miss the last mirror.
Furthermore, both of the above methods are applicable in one dimensional scanning only. For two-dimensional scans, an additional unit, which is either an identical or a mix with another device, must be provided for scanning in the other dimension, doubling cost and space requirements.
In U.S. Pat. No. 3,480,875 to R. V. Pole, disclosed is a scanning laser device, in which the spatial orientation of the laser beam in the resonant cavity is controlled by passing through a combination of a retardation plate and a Kerr cell inside the laser cavity. At a specific angle, as determined by the Kerr cell, loss is minimum for the laser beam, and therefore the laser beam will oscillate in that preferred direction. While this method allows scanning of large angles, the scan speed is limited by the laser build-up time, for which the laser beam intensity will be re-established at each new beam direction. Another drawback of this arrangement is the variation in the laser intensity during the laser build-up.
In U.S. Pat. No. 3,432,771 to W. A. Hardy, disclosed is another scanning laser, in which the optical cavity consists of a focussing objective, and spherical reflectors, or equivalent optics which consist of a lens and a plane mirror. The scan angle is magnified most effectively in an optical arrangement in which the two end reflectors form a nearly concentric cavity with the focussing lens at the center of focus. The drawback is that the cavity tolerates diverging beams to build up inside the cavity, as illustrated in FIG. 1 of the patent, hence the laser output has a high content of multiple transverse modes. By increasing the radius of curvature of the scan mirror and keeping its location fixed, the multi-mode content can be reduced, but the scan range will approach that of the actual scan angle with a possible small magnification factor. As suggested by its preferred embodiment with an electro-optical beam deflector, the scan angle will be only a few milli-radians if a near diffraction-limited laser beam is to be produced.
It would thus be desirable to have a scanner-amplifier unit which accepts a low energy laser pulse and emits an amplified laser pulse at a predetermined angular positions in two dimensions. The present invention provides such a unit.
The optimal surgical method for the cornea can be best appreciated from the characteristics required of the cornea to perform its important functions. The corneal surface is the first optical interface where all light enters into the eye and thereafter forms images at the retina. Corneal shape, degree of smoothness, and clarity all determine visual acuity and the contrast sensitivity of the vision system. Hence, the importance of the optical quality of the cornea cannot be over-emphasized.
The physical limits on the allowable surface roughness of the cornea can be understood by noting the following facts: human photo-sensors on the retina have a wavelength sensitivity range of about 380-850 nm in the optical spectrum; surface roughness exceeding half of the wavelength within the sensitivity range will act as light scattering centers; therefore, any inhomogeneity of the cornea surface or the inside stromal layer ideally should be kept at or below 0.2 microns to achieve an optically-smooth corneal surface.
The present invention recognizes that an optically smooth corneal surface and a clear cornea (including post-operative clarity) are all critical to successful refractive corneal surgery. The invention was developed with a particular view to preserving these characteristics.
The preferred method of performing a surface ablation of cornea tissue or other organic materials uses a laser source which has the characteristics of providing a shallow ablation depth (0.2 microns or less per laser pulse, and preferably 0.05 microns or less per laser pulse), and a low ablation energy density threshold (less than or equal to about 10 mJ/cm2), to achieve optically smooth ablated corneal surfaces. The preferred laser system includes a Ti-doped Al2O3 laser emitting from about 100 up to about 50,000 laser pulses per second, and preferably about 10,000 laser pulses per second. The laser wavelength range is about 198-300 nm, with a preferred wavelength range of about 198-215 nm, and a pulse duration of about 1-5,000 picoseconds. The laser beam cross-sectional area varies from 1 mm in diameter to any tolerably achievable smaller dimension, as required by the particular type-of surgery.
According to the present invention, each laser pulse is directed to its intended location on the surface to be ablated through a laser beam control means, such as the type described in a co-pending, commonly-owned patent application for an invention entitled xe2x80x9cTwo Dimensional Scanner-Amplifier Laserxe2x80x9d (U.S. patent application Ser. No. 07/740,004). The present invention also discloses a method of distributing laser pulses and the energy deposited on a target surface such that surface roughness is controlled within a specific range.
Additionally, the preferred apparatus for performing corneal surgery includes a laser beam intensity monitor and a beam intensity adjustment means, such that constant energy level is maintained throughout the operation. The location for the deposition of each pulse of laser energy relative to the surface to be ablated is controlled by monitor means such that eye movement during the operation is corrected for by a corresponding compensation in the location of the surgical beam. Provision for a safe and efficacious operation is included in the preferred apparatus, such that the operation will be terminated if the laser parameters or the eye positioning is outside of a predetermined tolerable range.
According to the present invention, various surgical procedures can be performed to correct refractive errors or to treat eye diseases. The surgical beam can be directed to remove cornea tissue in a predetermined amount and at a predetermined location such that the cumulative effect is to remove defective or non-defective tissue, or to change the curvature of the cornea to achieve improved visual acuity. Incisions on the cornea can be make in any predetermined length and depth, and they can be in straight line or curved patterns. Alternatively, circumcisions of tissue can be made to remove an extended area, as in a cornea transplant.
Although the primary use of the present invention is in ophthalmology, the laser ablation process can be applied in areas of neurology for microsurgery of nerve fibers, cardiology for the removal of plaque, and urology for the removal of kidney stones, just to mention a few possible uses. The present invention can also be useful for applications in micro-electronics in the areas of circuit repair, mask fabrication and repair, and direct writing of circuits.
The present invention provides an improved method of cornea surgery which has accurate control of tissue removal, flexibility of ablating tissue at any desired location with predetermined ablation depth, an optically smooth finished surface after the surgery, and a gentle surgical beam for laser ablation action.
The present invention also discloses a new method of reshaping a cornea surface with an optically smooth finish by depositing the laser energy in a prescribed pattern at predetermined locations. This is accomplished with high speed, precision control of the beam location, as disclosed in co-pending U.S. application Ser. No. 07/740,004 for an invention entitled xe2x80x9cA Two Dimensional Scan-Amplifier Laser.xe2x80x9d
The present invention also discloses a means to improve accuracy and reproducibility of eye surgery by adjusting the surgical beam direction to compensate for any eye movement during the surgical procedure. In addition, the surgical beam intensity, beam intensity profile, diameter, and location are monitored and maintained during the surgery.
Objects with Respect to the Inventive Method and Apparatus for Surgery
In accordance with the above discussion, these and other functions can be accomplished according to the teachings of the present invention, which provides a new and improved laser source, providing a gentler surgical beam and a shallower tissue etch depth than taught in the prior art.
It is another object of the present invention to provide an improved apparatus and method for removing organic materials from the surface of living or non-living objects. The present invention is specifically useful for the ablation of tissue on the cornea.
It is another object of the present invention to provide a method of ablating cornea or other organic materials to achieve an optically smooth surface.
It is another object of the present invention to provide new means of laser cornea surgery, with a new laser source emitting a large number of laser pulses (about 100 to 50,000 laser emissions per second), each of which etches a shallow depth (about 0.2 microns or less) of the cornea tissue.
It is another object of the present invention to provide new means of laser cornea surgery, with a new laser source emitting a wavelength of about 198-300 nm, with a preferred range of about 198-215 nm, and a pulse duration of about 1-5,000 picoseconds.
It is another object of the present invention to provide means of depositing surgical laser beam energy with a beam control as described in co-pending U.S. application Ser. No. 07/740,004, for an invention entitled xe2x80x9cA Two Dimensional Scan-Amplifier Laser,xe2x80x9d to achieve exact positioning of each laser pulse.
It is another object of the present invention to provide a gentler ablative surgery, with significantly reduced damage and trauma of the tissue or organic materials adjacent to the ablation site, in comparison to the prior art.
It is another object of the present invention to provide means to remove cornea tissue or other organic materials at predetermined locations, over predetermined areas, and with predetermined depths of ablation.
It is a specific object of the present invention to correct refractive errors, including myopia, hyperopia, and astigmatism, of the eye. It is another specific object of the present invention to correct refractive errors that may be spherically symmetric or asymmetric.
It is another object of the present invention to remove scars, tumors, and infected or opaque tissue on the cornea.
It is another object of the present invention to provide an improved method for performing a cornea transplant operation.
It is another object of the present invention to provide an improved method of making incisions on the cornea, to achieve correction of myopia and/or astigmatism.
It is another specific object of the present invention that the inventive methods be automated with computer control for accurate and safe operation.
It is yet another specific object of the present invention to provide control means for compensating for eye movement during an operation by making a corresponding adjustment of the surgical beam location.
Objects with Respect to the Scanner-Amplifier Laser Invention
The following objects are in accordance with the teachings of the co-pending, commonly-owned patent application for the invention entitled xe2x80x9cTwo Dimensional Scanner-Amplifier Laserxe2x80x9d (U.S. patent application Ser. No. 07/740,004).
An object in accordance with the present invention is to provide a scanner-amplifier unit which accepts a low energy laser pulse and emits an amplified laser pulse at a predetermined angular positions in two dimensions.
It is another object of this invention to disclose a construction of a high speed scanner-laser amplifier system, which has the capability of large scan angles, and the capability of emitting high quality, near diffraction limited laser beam. The scanner of the present invention can position a laser beam in two dimensions in a random access mode at high speed.
It is another object of the invention that the scanner-amplifier system generate ultra-short laser pulses of 1-500 picoseconds duration at a multi-kilohertz repetition rate, and that the energy of each laser pulses is amplified in a controlled manner to a desired level up to the damage level of the optical components.
It is another object of the invention that the laser medium is to be pumped by a plurality of laser beams in a longitudinal direction, such that high excitation density is achieved in the laser medium.
It is another object of the invention that the scanner-amplifier system can place an individual high energy laser pulse at a precisely intended angular location in a two-dimensional space.
It is yet another object of this invention to construct a Ti:Al2O3 laser with a high laser pulse rate, in the range of 1000 to 50,000 pulses per second, and with high average laser power, in the range of several watts or higher.
It is an object of this invention that each laser pulse has high peak power, and a short pulse duration, of sub-picoseconds to hundreds of picoseconds.
Still another object of this invention is to generate stable and high conversion efficiency in the second harmonic laser wavelength, which is used to generate population inversion in the Ti:Al2O3 laser medium.
It is an object of this invention to provide a novel method to attain high pump power in the second harmonic wavelength for the Ti:Al2O3 laser.
It is an object of this invention to propose a novel method to attain high pump power in an end-pumping configuration for the Ti:Al2O3 laser.
The preferred method for controlling the direction of the laser beam consists of a pair of scanning mirrors driven by piezo actuators. The mirror pair are driven in tandem. The scan angles of the mirror pair are summed and amplified by an optical arrangement. Two convergent spherical lenses of un-equal focal length are arranged between the scanning mirrors in such a way that a laser beam will be travelling inside the cavity in which the boundary is defined by the scan mirrors. For each round trip of the laser beam inside the cavity, the angle of the laser beam to an exit window increases as a multiple of the actual scan angles of the scan mirrors.
In accordance with this invention, the direction of the laser beam emitted from the scanner-amplifier system is controllable in two dimensions, at high speed, and with high precision.
In a preferred embodiment, the laser beam is generated by an amplifying means with a seeding laser pulses. Optical retardation plate, Pockels cell, and polarization dependent optical elements are used for the control of a seed laser beam and for directing that laser beam in the amplifier cavity. A laser gain medium is included in the cavity. Means for exciting the laser medium, and for generating multi-kilohertz, ultra-short duration laser pulses, are disclosed in the invention. Means for controlling the timing and the synchronization of the seed pulse, the pump source, and the amplified laser pulses inside the scanner-amplifier cavity are also provided.
It is an object of this embodiment to provide a means and method for combining a plurality of laser beams to provide a high power laser beam source.
It is another object of the invention to provide a combiner for combining a plurality of laser beams that does not require any form of specific polarization in any of the component beams. It is an object of such a combiner that it can form a beam bundle consisting of large number of beams in a small cross section.
It is yet another object of this invention to provide a novel method of combining a plurality of laser beams to provide a high power laser beam source for an end-pumping configuration of a laser beam. The combiner eliminates limitations imposed by the physical size of the beam steering optics and the optical mounts (an earlier method of beam combining relies on the direction of the linear polarization, and this method is limited to combining two beams only).
The details of the preferred embodiments of the present invention are set forth in the accompanying drawings and the description below. Once the details of the invention are known, numerous additional innovations and changes will become obvious to one skilled in the art.